Please get into technical terms. I work with computers so I'm not totally ignorant of technology. In fact I was working for Videotron Internet as a consultant abut 10 years ago.
VOD command are broadcasted exactly the same way as standard digital channel, meaning by QAM of 6 Mhz. 4 QAMs are reserved for this (only 3 in some region). One QAM can broadcast 38.8 Mbps (which is about 37.5 Mbps for the "real" data). Videotron encoded all there VOD stuff with a fix bitrate of 3.3 to 3.5 Mbps. Audio could be in 192 Kbps or 384 Kbps, which mean that the video bitrate is about 3.1 to 3.3 Mbps. 10 VODs command could be broadcast in the same time, within one QAM. So, it's 30 to 40 shows by "cell" (around 1000 doors).
When you looking at channel 900, it only start a software to let you choose what you want to see. When you order something, I don't know if the show is already in the hard drives of your local neighbourhood station but, anyway, it's connected by fiber optic, so, the time is very short before the show start. At this time, your terminal received the information about which QAM to tune and which program (numeric number) to watch. If you look at the page 1 of your status menu, when you are watching a VOD, you will see the frequency tuned by your terminal for this command. Before the show start, it's completly copied in your local station, on hard drive. So, you have full control over the broadcast, for one day. If you do nothing for more than 15 minutes, the program slot is freed for other VOD programs. If all the VOD programs slots are busy, you will get a error code but this is really rare.
By the way, there is some HD show now. They seemd to be encoded to take the place are 4 SD programs.